Harnessing solar energy as a substitute for combustion-based energy production continues to generate interest. Concentrated solar power (CSP) systems use reflective panels to direct and concentrate solar energy onto a receiver, where the solar energy heats a heat transfer fluid (HTF) that ultimately conveys heat to a heat engine, typically a steam turbine. Various CSP system designs exist, including parabolic trough, power tower, Fresnel reflectors, concentrated thermoelectric devices, and dish Stirling systems. Other solar energy systems can include solar thermochemical cycles, solar biomass gasification, high temperature catalysis, solid-oxide fuel cells, and solid oxide electrolyzers.
Since solar energy collection is obviously limited by daylight availability and cloud cover, the efficiency and economic viability of CSP systems can be greatly enhanced through use of thermal energy storage (TES). Using TES, the energy conversion system can operate over an extended timeframe, even during periods of extensive cloud cover or at night, or energy production can be shifted to times of greater demand. A variety of TES systems have been developed, including various two-tank systems and single-tank thermocline systems, and these TES technologies offer variations depending on whether the HTF and the thermal storage medium are the same or separate materials.
An important consideration in developing a viable TES system is the type of thermal storage medium that will be utilized. A conventional organic HTF is a eutectic mixture of biphenyl and diphenyl oxide (sold commercially under the THERMINOL tradename). However, THERMINOL fluids have very low thermal conductivity and a relatively low temperature threshold as stable liquids, and are therefore poorly suited as a thermal storage medium. Molten salt mixtures, such as mixtures of sodium nitrate and potassium nitrate (so-called “Solar salt”), have gained more recent attention as thermal storage media. Such mixtures exhibit greater thermal stability and thermal conductivity than THERMINOL materials, and have been successfully used in TES systems, particularly the parabolic trough system of the Andasol solar power station and the power tower system used in the Gemasolar plant, both in Andalusia, Spain. Indeed, the Gemasolar power tower system became the first commercial CSP system to produce an uninterrupted supply of electricity for 24 hours during the summer of 2011. Other materials have also been proposed as thermal storage materials in TES systems, including high-temperature concrete or castable ceramic materials, molten metals, composite nanofluids (e.g. nanoparticles dispersed in a liquid), high pressure gas, molten glass, and phase change materials (PCM).
The thermal storage media proposed to-date suffer from certain drawbacks. For example, the molten salt mixtures presently used in commercial CSP plants exhibit a melting point over 200° C., which means special care must be taken to avoid freezing along the fluid pathway. PCM materials require complex systems, usually involving multiple heat exchangers and multiple PCMs that melt at different temperatures. Concrete or ceramic solid storage structures are limited by the heat transfer rate between the HTF piping and the solid structure. Additionally, concrete structures are limited by the composition and currently also limited to temperatures below 700° C.
Accordingly, there is a continuing need in the art to provide thermal storage media capable of efficiently operating within TES systems adapted for use in CSP plants.